Can I use designs that I purchased from Adobe stock to use on Merch by Amazon t-shirt?

For merchandising, you need extended licencing: https://stock.adobe.com/license-terms#extendedLicenses.

So if you acquire an extended licence of a design, you can use that design to print on a T-shirt and sell that.

Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).